Archives of Number Theory List: Archives of postings to the Number Theory mailing list (NMBRTHRY).
FermatNumbers: A group for aggregating all the known properties of Fermat Numbers F(m)=2^2^m+1 in one place. Links to distributed search and lists. Archive available to non-members.
goldbach: Discussion list for the Goldbach Conjecture.
numbertheory: Mailing list covering recreational and serious aspects. [Requires free Yahoo membership.]
PrimeForm: Discussion group for the primality-testing program, PrimeForm. Includes links to the PrimeForm program's home page, contact and download information.
Primenumbers: Discussion list for those who enjoy prime numbers, primality testing, and closely related subjects.
Somos Sequence Mailing List Archive: Maintained by Jim Propp.